Since the problem is not due to the sources, make sure if you have any corrupted script on globalevents/creaturescripts. Might be because of login, logout, death, or something else that causes.
As you said it's usually happening between 15-30 minutes aprox., then look for any last changes made in globalevents. You can also try to remove spawn file in world and see if it stays online for over 30 minutes, if it does then there has to be wrong with some monster/npc.